Disk Utility is an app that comes with your Mac. It’s tucked away in the Utilities folder, which is found inside the Applications folder, but is easy to locate using Spotlight - which you trigger by pressing Command+Space Bar. Here’s what you can use Disk Utility for:

  • Disk Utility is use to erase, format or manage internal disks and external storage devices.
  • Disk Utility can also be used to add and manage volumes, or if you want to organize your date into partitions.
  • Disk Utility can be useful if you are working with RAID sets. Combining multiple disks into a RAID set that acts as a single disk can increase performance, reliability and storage space.
  • You can use Disk Utility to create a disk image of files that you want to move to a different computer, archive or back up.
  • Disk Utility is used to diagnose and fix problems with a disk or damaged volumes.

Disk Utility was overhauled in Mac OS X El Capitan when it got a redesign and many of the features evolved or were removed.

There are also a few changes in macOS High Sierra thanks to the new file format Apple has introduced: APFS (more on that below).  

Why use Disk Utility

You might want to use Disk Utility it if you experience the following:

Your computer won’t start up. (Read: How to fix a Mac that won’t turn on).

An external device won’t work. (Read: How to access an external drive that’s not showing up on a Mac).

  • Your apps are quitting unexpectedly.
  • You have a corrupted file.
  • You need to mount, unmount, or eject a disk.
  • You need to change the file system - for example to enable or disable journaling.
  • You think that there is a problem with your startup disk.  
  • You need to repair a disk that is damaged.
  • To erase, format or partition a disk.
  • Disk Utility used to be used to verify and repair permissions but since El Capitan this is unnecessary.

How to find out if your disk is faulty using Disk Utility

If you think there is a problem with the drive inside your Mac or an external storage device, you can use the First Aid feature in Disk Utility to check.

The First Aid feature in Disk Utility will run various checks and if it detects an issue with your disk it will repair it.

Note: Disk Utility had a bit of a makeover in OS X El Capitan and as a result the way that you repair a disk has changed slightly. The steps you need to take will depend on the version of MacOS you are running. We’re going to assume you are running a version that is newer than El Capitan since that version of Mac OS X launched in September 2015.

Here’s how to run First Aid on your Mac to check if there is a problem with your disk:

  1. Open Disk Utility.
  2. Select your device that you are having problems with from the sidebar.
  3. Click on First Aid.
  4. Disk Utility will check the partition maps on the disk and check each volume. If you run First Aid on a volume only, Disk Utility will just verify the contents of that volume.

If it finds a problem with the disk, Disk Utility will attempt to repair it.

If Disk Utility tells you the disk or volume is about to fail, you should back it up and replace it. Disk Utility will not be able to repair it.

If Disk Utility reports that the disk is OK that means there isn’t a problem with the disk.

How to repair a disk with Disk Utility

How you repair a disk depends whether it’s your Mac's Startup Disk (the disk that you Mac runs from, where the operating system is installed), or a separate disk that's plugged into your Mac. If it’s your Startup Disk jump to the next section.

  1. Open Disk Utility (in Applications > Utilities, or cmd+space Disk Utility).
  2. Select the volume you wish to run First Aid on. This could be a external hard drive (if it’s your own Mac hard drive you will need to jump to the next section).
  3. Click on First Aid.
  4. Click Run. This will start the verification and repair process.
  5. When Disk Utility has run it’s checks you will see a drop-down sheet showing the status. You can click on the triangle at the bottom to see more information.
  6. If no errors were found you will see a green tick at the top of the drop-down sheet.
  7. If there were errors Disk Utility will attempt to repair them. (In older versions you had to manually choose Repair Disk).

If Disk Utility is unable to repair the drive, or it believes that the disk is about to fail it will warn you. Should this be the case you should back up your data before it’s too late. Read this article on backing up your Mac.

You may also find this useful: How to recover and delete data from a failed hard drive, SSD or external drive.

How to repair your boot disk/startup disk with Disk Utility

You can run First Aid on your startup drive as above, but if Disk Utility finds any errors it won’t attempt to fix them.

If you need to repair your Mac’s startup drive (the boot volume) you won't be able to as Disk Utility can’t repair the mounted volume (the one that everything is running from). In older versions of Disk Utility you will see that the Repair Disk option is greyed out.

In this case you need to start up your Mac in Recovery Mode and repair the disk from there. This way things can run from the Recovery HD volume that was created when MacOS was installed. (Note if you have a Fusion Drive things are even more complex).

  1. To start in Recovery Mode press cmd+R when you start up your Mac.
  2. Once your Mac has booted up you will see a Utilities screen. Choose Disk Utility.
  3. Select the disk you wish to repair from the menu and clink on First Aid. As above Disk Utility will run its checks and try and repair if it can.
  4. The repair process may take a while.

How to repair disc permissions

When Apple released El Capitan in 2015 it removed the ability to repairing disk permissions.

The removal of the feature probably indicates that repairing permissions didn’t really do a whole lot of good.

It is still possible to repair permissions using Terminal, but we won’t go into that here, following Apple’s lead and assuming that it won’t do any good and might actually cause more issues.

However, if you are using Mac OS X Yosemite or earlier, and a file's permissions are no longer set correctly, software that uses the file might not work correctly. If that’s the case, here’s how to fix permissions:

  1. Open Disk Utility, which is in the Utilities folder of your Applications folder.
  2. Select the startup disk from the list of volumes.
  3. Click the First Aid tab.
  4. To check permissions, click Verify Disk Permissions. To repair permissions, click Repair Disk Permissions.

How to create a Disk Image using Disk Utility

Disk Utility is able to create a disk image of contents of a folder which you can then transfer to another Mac, an archive, or any location that doesn’t accept folders.

It’s similar to compressing the folder into a zip archive, but the benefit is that you can not only use disk image compression to save space, but can also take advantage of Apple’s encryption for the disk image.

Follow these steps to create a disk image using Disk Utility.

How to format a drive using Disk Utility

There are a number of reasons why you might want to format a drive. Perhaps you want to wipe your startup drive so you can perform a fresh install of OS X, maybe you want to encrypt an external storage drive that you use for work, or you might wish to create a partition for Windows or a different version of the Mac operating system.

We have a step by step guide to formatting a drive on the Mac here

Fix problems with an external drive

Here's what to do if you plug a hard drive or flash drive in to your Mac and doesn't see it and won't let you access the data: What to do if a hard drive won't mount. 

Partition a disk

You may wish to partition a disk if you want to install multiple operating systems. How you partition will depend on whether you are running High Sierra or an earlier version of macOS or Mac OS X.  There’s more information about how to partition a disk here

We have a guide to how to partition a Mac here.